In a communication system a communication network is provided, which can link together two communication terminals so that the terminals can send information to each other in a call or other communication event. Information may include speech, text, images or video.
Modern communication systems are based on the transmission of digital signals. Analogue information such as speech is input into an analogue to digital converter at the transmitter of one terminal and converted into a digital signal. The digital signal is then encoded and placed in data packets for transmission over a channel to the receiver of another terminal.
One type of communication network suitable for transmitting data packets is the internet. Protocols which are used to carry voice signals over an Internet Protocol (IP) network are commonly referred to as Voice over IP (VoIP). VoIP is one of the protocols used for routing of voice conversations over the Internet or through any other IP-based network.
It is known to connect more than two terminals via a communication system in a conference call. In a conference call, a group of participating terminals may be connected together via a host. The host is arranged to collect incoming signals from each participating terminal and combine the signals before sending the combined signals to the participating terminals.
Reference is made to FIG. 1 which shows a group of participating terminals 110, 112, 113, 114 and 116 connected via the internet 104. In this arrangement a host 115 resides in a participating terminal 116. Alternatively the host may be provided by a host sever.
In order to combine the signals it is necessary for the host to decode the signals and mix the decoded signals before transmitting the combined signals to each of the participating terminals.
When hosting a conference call the number of signals that are decoded is proportional to the number of participating terminals. For conference calls with a large number of participating terminals decoding each signal provided from the participating terminals places a large burden on the resources of the host.
A participating terminal may be a personal computer or a mobile phone with limited processing resources. Therefore the burden placed on the resources of the host can be a particular problem if the host resides in one of the participating terminals. For example if the CPU (Central Processing Unit) usage of the host is too high this will result in poor sound quality during the conference call.
In order to reduce the burden on the resources of the host it is known to select only some of the incoming signals to be decoded. In most cases only some of the signals provided from the participating terminals will need to be mixed as in general the only some of the signals will contain speech.
One known method of reducing the number of signals that the host is required to decode is achieved by transmitting a bit in each data packet to indicate whether the signal contains speech. In this case the host is arranged to examine the bit in the packet before fully decoding the packet. Only packets that are indicated to include speech are then decoded by the host.
However this method is of limited benefit as the host is still required to decode each packet that contains speech. The host may determine that the decoded packet of a signal is not required in the mixed signal if for example the speech contained in the packet is too quiet. As such the number of signals that have been decoded will exceed the number of signals included in the mixed signal.
An alternative method of reducing the number of signals that are decoded by the host requires each signal to be encoded with the same encoding scheme. In this case the host may use knowledge of the encoding scheme to analyse the encoded bit stream to estimate the speech strength in each signal. The host may then determine which signals should be decoded from the relative speech strengths of the signals.
However this method is limited as it requires each participating terminal to use the same encoding scheme. Furthermore it is necessary for the host to apply a detailed analysis to the encoded bit stream which also uses significant resources.
It is therefore an aim of the present invention to overcome the above identified limitations of the prior art. It is a further aim of the present invention to provide a method of efficiently processing signals in a conference call without the use of complex computational methods.